1. Field of the Invention
This invention relates to a reel shaft in a magnetic recording and/or reproducing apparatus for performing recording and/or reproducing operation with respect to a magnetic tape cassette installed therein.
2. Description of the Prior Art
A typical example of conventional reel shaft employed in a digital audio taperecorder (DAT) unit is illustrated in FIG. 5 through FIG. 9. A pair of cylindrical reel hubs 3, 3 (only one of which is shown in the drawings) are freely rotatably mounted on a casing 2 of a DAT cassette 1, around which a magnetic tape T is wound. The reel hub 3 is made hollow in the axial direction to provide a reel shaft inserting aperture 4. The aperture 4 is provided with a plurality of projections 5, equally spaced apart one another, which extend in parralel with the axis O of the reel hub 3. More particularly, as best seen in FIG. 6, each projection 5 includes a center walls 5a confronting the axis O and a pair of side walls 5b, 5b tapered by a predetermined angle with respect to the axis O. The projection 5 has thus a trapezoidal cross-section when taken along a plane perpendicular to the axis O.
The reel shaft 7 arranged on the DAT unit for winding up the magnetic tape T is driven by a drive shaft 8 rotatably supported on a stationary shaft 10 planted on a mount chassis 9. The drive shaft 8 has a regular hexagon in a cross-section taken along a plane perpendicular to its axis P. A disc 11 is secured to the bottom of the drive shaft 8 for transmitting rotation of a motor (not shown) therethrough to the drive shaft 8.
A reel hub driving member 12 has in main a lower cylindrical section 12b and an upper truncated conical portion 12a with its diameter being decreased toward the top. The reel hub driving member 12 is made hollow to provide an aperture 13 of a regular hexagonal cross-section for engagement with the drive shaft 8. The top of the driving member 12 is covered by a cap 14. A coil spring 15 is disposed between the disc 11 and the bottom of the driving member 12 to normally urge the latter toward the uppermost position. With such arrangement, the driving member 12 will be rotated together with the drive shaft 8 but relative displacement therebetween in the axial direction will be permitted against depression of the coil spring 15.
A plurality (or three in this example) of projections 16, equally spaced apart one another by 120.degree. angle, are integrally formed or secured to the outer periphery of the reel hub driving member 12. Each projection 16 includes a pyramid-shaped top section 16a attached to the upper truncated conical section 12a of the driving member 12 and having faces tapered with respect to the axis P of the reel shaft 7 and a lower spure columnar section 16b attached to the lower cylindrical section 12b and extending in parallel with the axis P. The lower section 16b has a configuration corresponding to the projection 5 of the reel hub 3.
After the DAT cassette 1 has been loaded into a predetermined position in the DAT unit, the reel shaft 7 passes through an insertion hole 6 formed in the underside of the casing 2 to enter the aperture 4 of the reel hub 3. Thus, the projections 5 of the reel hub 3 is brought into engagement with the projections 16, as best seen in FIG. 7. Configuration of the upper sections 12a and 16a of the driving member 12 and the projections 16 respectively will ensure smooth insertion of the reel shaft 7 into the aperture 4. Since the projections 5 and 16 have substantially the identical configuration and extend in parallel with the axes O and P respectively, engaging faces 16b of the projections 16 will be in touch entirely with engaging faces 5b.sub.1 of projections 5 as far as these axes O and P coincide with each other. With such engagement, the reel shaft 7 is driven by the motor to rotate in a direction shown by the arrow A, which causes the reel hub 3 to rotate to thereby reel up the magnetic tape T into the reel hub 3.
It has been found that the above-described prior art mechanism is duly applicable to a DAT unit of horizontal type in which the axes O and P of the reel hub 3 and the reel shaft 7 respectively extend in a vertical direction. However, in another type DAT in which the axes O and P extend in a horizontal direction, it will practically be impossible to provide concentric relationship in position between the reel shaft 7 and the reel hub 3 due to gravity. More specifically, since the outer diameter defined by the projections 16 of the reel shaft 7 is somewhat smaller than the inner diameter of the aperture 4, because of its own weight and weight of the magnetic tape T, the reel hub 3 would tend to be lowered relative to the reel shaft 7, thereby producing an offset between the axes O and P in the DAT cassette loaded position, as shown in FIG. 9. Eccentric rotation of the reel hub 3 would result in wow/flatter of the magnetic tape T.
Suppose another case wherein the reel hub axis O extends in parralel with the reel shaft axis P. However, in such case, the engaging face 15b.sub.1 of the hub projection 5 is secured tapered with respect to the reel hub axis O, engaging face 16b.sub.1 of the shaft projection 16 does not contact with the engaging face 5b.sub.1 by means of each face. While rotating the reel shaft 7 under such tapered condition, there would appear increased distortion in the axial direction of the reel hub 3 in winding of the magnetic tape T. Much distorted winding of the magnetic tape T would tend to come into contact with the inner wall of the casing 2 of the DAT cassette 1, resulting in failure of exerting a predetermined tension to the magnetic tape T.